I am a project manager for a flooring firm in MN and am currently monitoring the RH on a slab for a project we will be installing VCT on.

I have 4 tests in currently, and they have been in for 3-1/2 weeks.

My question has to do with how much one can expect the RH readings to vary day to day? On Friday, my readings were 90%/96%/99%/88% and today, Monday, they were 93%/96%/99%/91%. So tests 1 and 4 both swung by 3 percentage points. I have some concerns that test #1 may have been compromised, as 2 weeks ago it went from 93 to 89 to 81 and then up to 93 all in a weeks time. Then it stayed at 93 for a week, then did the 88 to 91 swing over the weekend.

The adhesive we are using requires a 90% reading.

I will also note that the pH readings have been at between 7-7.5 the entire time. I also did one calcium chloride test near my RH test that has been reading 99% the entire time, and that came in at 6.87#/1000sf/24 hrs.

I know when I report the readings to the GC they are going to question why the readings were ok on Friday, but have now swung back up into the unacceptable range. If anyone knows were I can find more documented information on this I would appreciate that as well.

You bring up some great points here. Two variables you have not told us of though are:

1) The ambient conditions in the room, RH% and temp.
2) Concrete temp each time you read the RH% in the concrete

These are very important to know in answering the question appropriately. I can paint a broad brush and try to give some input.

1) Friday to Monday variance-Typically, the jobsite heat is turned down and or off over the weekend because there are typically no trades working. If the temperature is altered drastically over this time period, the RH% will respond accordingly. All things being equal, RH% and Temp are inversely related.
2) Another thing to look at is have mudders and painters been in the area over this time period. Both will attribute to higher than normal RH% in the area due to moisture and heat. This can impact the concrete readings.
Concrete is like a sponge and tries to emulate the environment it lives in. Taking two steps forward (lower RH%) and on step back (higher RH%) will happen as conditions change in the room during construction.
If you can get the two bits of information I first stated, the forum may be able to help more.

Thanks. I just spoke with Julie on the phone as well, and did get more information to help me better understand what is going on, and to help combat the questions being posed by the GC.

I do not have the ambient RH and temp, but the temp of the concrete in both of the holes that had changes from Friday to Monday had dropped.

The project is fully complete (painted, ceilings are in) except for the VCT, so the only work occuring in the building over the last week has been some telecommunications type electrical work and installation of hardware.

I will have to find out about the heat - I would suspect that they are turning it down over the weekend, but not positive.

I think the biggest issue with this project is that they poured concrete on March 6th and we started testing it 6 weeks later. I am not surprised at all that the readings are high. They have the expectation that the drying should happen overnight, which we know is not the case. The majority of the slab is 4" thick and only about 8 weeks old.

Erica,
One point (and I just watched a gentleman do this today) is to not let the reader sit in the probe. You want to insert the reader and remove it within a few seconds and read the results. What could be happening is the thermal mass of the reader is draining into the probe and changing the probe's reading. The best way I can explain this is imagine the probe is acclimated in a concrete slab at 70 degrees and you bring in a cold reader from your vehicle while enjoying your -10 degree Minnesota weather. That freezing reader will cool off the probe as it sits in the hole and throw the RH reading off the charts. I use this extreme scenario to illustrate the point. Within more modest temperature differences a quick plunge of the reader into the probe will not affect the reading.

Do you have a psychometric chart available?
We have tried to correlate the readings we see with a standard psychometric chart but they don't exactly follow the chart. But you would get a pretty good indication of what is happening with the temperature vs humidity by checking your numbers on the chart.

The temperature of the concrete is an important aspect of determining acceptable RH (which is why we would love to be able to follow a psychometric chart). The ASTM states we must be at service temperature, but how much will the RH change if we measure our slab at 75 degrees and we expect service temperature to be 65 degrees? The psychometric chart says the amount of moisture that gives us 72%RH / 75 degrees, is the same as the moisture in 100%RH / 65 degrees!

Unfortunately we haven't been given the green light to use the psychometric chart for calculating RH at service temp yet, but I want you to be aware of how much temperature can affect RH so you can make judgements on what you are seeing when you take readings. If you see a couple of degrees drop in temps, I'd expect to see anywhere from a couple of percent or more of rise in RH.

Kind of Pat,
The ASTM says the probe must acclimate to the temperature of the concrete. The temperature at the 40% depth may vary from the surface temperature.

With the Wagner Rapid RH the sensor is left inside the concrete to acclimate. Your most accurate reading is the initial reading when inserting the reader.

With removable probes in sleeves, the sensor is in the probe and that sensor needs to acclimate to the concrete temperature which can take quite a long time. Many manufacturers recommend waiting an hour or more for each reading. Even then, think of when the slab is on the ground and the internal temperature is 67*F and you have an aluminum probe sticking up into the room atmosphere which has the heat on and blowing on that probe at 80*F. That heat can change the reading of that probe! Then the next time you read that probe maybe the heat is off, or the cooling is on. It has been my experience that I could get wild swings in my RH measurements using removable probes and leapfrogging from hole to hole, from day to day, or when the sun shines on a probe or it is shaded.

AH, well this is where I get confused because of the difference between the Wagner system and the others (I've only used Protimeter).

With the Proti system, the thing you're inserting into the concrete is just a "sleeve", and the probe is the thing attached to the handset on a cord, and that probe constantly needs to be calibrated/ kept at the right temp/ etc.

With the Rapid RH, the thing you're inserting into the concrete IS the "probe" and the little orange reader thingy kind of takes the place of the handset...hence no need to calibrate the probes, because they're all expendable..

Is that about right, if you'll pardon the excessively layman-like termage?

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